This invention generally relates to synthetic resins. More specifically, it relates to methods for preparing poly(arylene ether) polymers useful in high-temperature applications.
Poly(arylene ether) resins are commercially attractive materials because of their unique combination of physical, chemical, and electrical properties. The resins are usually characterized by a desirable combination of hydrolytic stability, high dimensional stability, toughness, heat resistance, and dielectric properties. They also exhibit high glass transition temperature (Tg) values, typically in the range of about 150° C.–215° C., as well as very good mechanical performance.
Poly(arylene ether)'s such as the family of polyphenylene ether (PPE) resins are often used in combination with other polymers, to further enhance the attributes of the ultimate polymer product. For example, PPE resins are often combined with polyamides. The resulting molded products exhibit the most desirable properties of each material, e.g., excellent heat resistance and dimensional stability from the PPE, and excellent strength and chemical resistance from the polyamide. End use applications for structural materials of this type include computer housings, automotive panels, and the like.
In recent years, it has been desirable to further increase the high-temperature capabilities of materials like the poly(arylene ether)/polyamide blends. For example, automotive panels made from such materials need to withstand relatively high paint oven temperatures when the parts are coated. One means of increasing the temperature capabilities of the polymers has been to employ certain copolymers for the poly(arylene ether) component. For example, European Patent Application 627,466 describes the preparation of poly(arylene ether) copolymers of 2,6-dimethylphenol and 2,3,6-trimethylphenol. Copolymers of this type are often characterized by relatively high Tg values, e.g., in the range of about 225° C. to about 230° C.
A variety of methods for preparing and isolating poly(arylene ether) homopolymers and copolymers are well-known in the art. Many are described in the background section of U.S. Pat. No. 6,407,200 (P. Singh et al). One well-established method for isolating the polymers involves combining the polymerization reaction mixture with an anti-solvent such as methanol, and then filtering the resulting precipitate.
Many variations on the general technique of precipitation and filtration have been developed over the years. For example, the European application mentioned above describes the isolation of a poly(arylene ether) copolymer by reverse-precipitation with acetone, and filtration. Other techniques also involve pre-concentration steps, wherein a portion of the reaction solvent is removed prior to precipitation steps.
The isolated polymer is then subjected to additional, conventional processing steps. For example, the polymer can sometimes be re-slurred with the anti-solvent, then filtered again, and then washed with additional solvent. Conventional solid/liquid separation techniques include filtration, solid bowl centrifuges, gravity settling, and the like. After these additional steps, the polymer is usually dried by various procedures. For example, drying can be carried out at elevated temperatures, under atmospheric pressure or reduced pressure, using various types of industrial drying equipment, e.g., pneumatic conveying dryers, screw-conveying dryers, or fluid bed dryers. The resulting product, in powder form, is then typically transported to other locations for storage or compounding. In large commercial plants, transport is often undertaken by pneumatic systems, through a network of pipes.
The procedures described above are often suitable for efficiently producing high yields of the desired poly(arylene ether) product. However, there are some drawbacks to the processes. For example, the poly(arylene ether) powder produced by these techniques sometimes includes an undesirably high proportion of powder “fines”. As used herein, the term “fines” refers to solid particles having a particle size less than about 38 microns (micrometers).
Fines can be responsible for a variety of problems during the processing of the polymer. Their presence may be associated with losses of the poly(arylene ether) during the filtration and drying stages. Fines tend to stick to the processing line filters, where they can cause clogging and an excessive pressure drop. (Filter pressure drops may trigger alarms which shut off the powder transport through a transport line, e.g., from a resin silo to a silo in a compounding area). In general, the presence of high powder fines can make it difficult to efficiently separate the polymer powder from gas in the drying and transport systems, resulting in the accumulation of fines in vent systems, and possible dust emissions into the atmosphere. Removal of the fines from the filters can be a difficult and time-consuming task. Moreover, the presence of fines can create dust explosion hazards if powder-handling involves contact with air, thus requiring the installation of expensive safety equipment.
In addition to transport and flow problems, the presence of fines can also result in significant problems during extrusion and molding of the polymer product. For example, though they constitute part of the solid powder composition, fines often do not have the minimum, solid mass and density necessary for proper flow, into and through an extruder. Thus, the appropriate shearing forces for extruding the polymer product may not be attainable when too many fines are present.
With these concerns in mind, attempts have been made to reduce the level of fines in a solid polymer product, such as the poly(arylene ether) materials. One strategy for this objective involves changing the process conditions under which the polymer product is made. For example, the previously-referenced U.S. Pat. No. 6,407,200 describes a method of preparing poly(arylene ether)'s, wherein a portion of the reaction solvent is removed after the catalyzed oxidative reduction step. Removal of the solvent portion leaves a concentrated solution of the polymer product. This solution is subsequently combined with an anti-solvent (as mentioned above), to precipitate the desired polymer product. The inventors associated with the referenced patent discovered that if the temperature of the concentrated solution was elevated to specified levels immediately before combination with the anti-solvent, the generation of the undesirable fines could be reduced.
U.S. Pat. No. 6,407,200 documents a considerable innovation in poly(arylene ether) preparation and processing—especially in the area of fines reduction. However, additional improvements in this area of technology would also be of great interest—especially in an era when the demands for ever-higher product yields and product quality are present. Thus, there remains a need for improved processes for preparing poly(arylene ether)'s.
The new processes should result in greater reductions in fines-content for the powder products, and an improvement in powder flow behavior. The processes should also be compatible with the production of the high-temperature poly(arylene ether) copolymers discussed above, which are also in great demand today. Moreover, the processes should be economically adaptable to a large-scale manufacturing facility, e.g., without significant changes in facility structure or processing requirements. Furthermore, the overall properties for products molded from the polymer material should be substantially maintained.